Several different measures have been proposed for straightening up the leakage flux, for example by locating, adjacent to the windings, bodies of high permeability, wire-wound coils traversed by the winding current, or shields of electrically conducting material (see, e.g. U.S. Pat. Nos. 3,142,029 and 4,012,706). However, these measures only result in a limited reduction of the edge current density.